Obesity is a major source of medical morbidity (hypertension, diabetes, hyperlipidemia); in the U.S., approximately 25% of adults and 20% of children are obese. Although this disorder must ultimately be due to an imbalance in food intake and energy expenditure, intensive clinical and animal investigation has failed to elucidate the molecular-physiologic mechanism for obesity in any single instance. Mice homozygous for either the ob or db gene display a metabolic phenotype which closely resembles that of the obese human. Animals homozygous for either mutation, whose genes lie on different chromosomes, are massively obese and frequently weight 3 times as much as lean littermates by 10 months of age. These animals also demonstrate: enhanced metabolic efficiency, hyperphagia, peripheral insulin resistance and diabetes mellitus. Many attempts have been made to identify the specific protein or pathway responsible for the phenotype of these animals. However, it is unclear which differences (if any) are primary (i.e. directly related to the gene) and which are secondary to the profound metabolic alterations it produces. This very inability to disentangle primary from secondary events is the most compelling argument for the use of reverse genetics in delineation of the specific lesion in these animal models. The ob and db genes, which appear to control separate aspects of a single metabolic pathway or control loop related to energy homeostasis will be genetically mapped, and the gene having most closely linked RFLPs selected for molecular cloning and characterization. Employing techniques of "reverse genetics", the ob and db genes will first be mapped genetically on large backcrosses relative to appropriate genomic probes, and then isolated by physical methods including pulsed field gen electrophoresis, cosmid clones and/or yeast artificial chromosomes. Gene identity will be assessed by screening for tissue- specific expression of candidate sequences, and their subsequent "complementation" of the mutant phenotype of transgenic insertion. Our proximate aim is the localization and preliminary characterization of the genomic lesions in one of these forms of mouse obesity. Identification of the genomic sequences (or closely linked RFLPs) causally related to these animal obesities may permit direct mapping of these genes on relevant human pedigrees. Such linkage analysis would permit rapid assessment of the relevance of any mouse genomic sequence to obesity in man. Most importantly, the basis for at least one type of genetic obesity will be understood at the level of gene action.